1. Field of the Invention
The present invention relates to a point to multipoint device for use in a wireless network to provide wireless communication with a plurality of telecommunication units, to a method of operating such a device, and to a wireless network comprising a plurality of such point to multipoint devices.
2. Description of the Prior Art
Point to multipoint devices within a wireless network may take a variety of forms. For example, such a point to multipoint device may take the form of a relay node or repeater used to propagate data within the wireless network. Such relay nodes or repeaters typically amplify and forward or decode and forward received signals within the wireless network. Another example of a point to multipoint device would be a base station associated with a cell of the wireless network for communicating over wireless links with a number of subscriber stations and/or relay nodes in the cell.
A wireless network infrastructure typically subdivides a geographic area into mutually disjoint regions called cells. Associated with each cell are one or more base stations (BSs) that communicate via radio signals with a number of subscriber stations (SSs) located within the same cell. The transmission path from the BS to the SS is known as the forward link or downlink communication path, whilst the transmission path from the SS to the BS is known as the reverse link or uplink communication path.
In one implementation, the BS may be connected to a telephone network and exists to relay messages from SSs in the cell controlled by the BS to the telephone network, and vice versa. By this approach, an item of telecommunications equipment connected to an SS may make an outgoing call to the telephone network, and may receive incoming calls from the telephone network.
However, such a wireless telecommunications system is not restricted to use with telephone signals, but could instead, or additionally, handle any other appropriate type of telecommunications signal, such as video signals, or data signals such as those used for transmitting data over the Internet and in order to support recent technology such as Broadband and video-on-demand technologies.
Within such a wireless network, a measure of the received signal quality is the Signal to Interference and Noise Ratio (SINR). For a given SINR, the receiver (for example the receiver provided at an SS) can request a suitable Modulation and Coding Scheme (MCS) that will maximise the data rate and at the same time ensure an acceptable Quality of Service (QoS). The Frame Error Rate (FER), i.e. the percentage of blocks of data that are received in error, is frequently used as a measure of the QoS. If a block of data is incorrectly decoded, then the receiver will inform the transmitter (for example the transmitter at the BS when considering the receiver at the SS) to resend the data. Whilst such a scheme is necessary to maintain an acceptable QoS, data repetition has the drawback of reducing the overall system throughput.
Cell sectorisation is a well-known technique for increasing the system capacity, system capacity being a measure of the ability of a network to serve and sustain simultaneous users. In cell sectorised layouts, the area within a cell is, under ideal situations, sub-divided into a number of non-overlapping regions called sectors. The sectors within the same cell are served by the same BS, or by different BSs (one per sector). In such sectorised layouts, the point to multipoint device may be considered to be the entire base station, or the sector specific logic, whether that be provided as a physically separate base station or as a part of a base station covering the entire cell. Sectorisation is generally implemented by employing highly directional antennas that concentrate the radiated energy within a sector. FIG. 1 shows a cellular network consisting of seven cells, with each cell comprising three sectors. Hence, by way of example, the cell 10 illustrated in FIG. 1 is served by a base station 20 which can provide separate beams to cover the three sectors 30, 40, 50 provided within the cell 10.
Typically, a BS may need to communicate simultaneously with multiple SSs within a sector or a cell. Typically, such simultaneous communication can be achieved by defining multiple communication channels that can be arranged to utilise the radio resource of the wireless network. For example, in a “Time Division Multiple Access” (TDMA) system, a particular frequency channel can be partitioned in the time domain, such that a number of different signals can be transmitted in different time slots, the time slots forming multiple communication channels utilising the particular frequency channel. As another example, in a “Frequency Division Multiple Access” (FDMA) system, a band of frequencies may be partitioned to form a number of communication channels of particular frequencies, thereby enabling multiple signals to be transmitted over the radio resource. In a combined TDMA/FDMA system, such as used in WiMAX systems, a combination of time/frequency slot is used to define separate communication channels. WiMAX systems are based on the IEEE 802.16 standards that provide high-throughput broadband connections over relatively long distances.
As another example of a mechanism that can be used to establish multiple communication channels within a radio resource, in a “Code Division Multiple Access” (CDMA) system, signals may be transmitted over the radio resource on a particular frequency channel, and this frequency channel may be partitioned by applying different orthogonal codes to signals to be transmitted on that frequency channel. Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal communication channel utilising a particular frequency channel.
The total number of resources (i.e. channels) in a wireless network is limited. In order to increase the system capacity it may be necessary to use the same channel in different cells and/or sectors. This is known as channel re-use. The cells or sectors that use the same set of channels are known as co-channel cells or sectors, and the interference generated as a result is referred to as Co-Channel Interference (CCI). CCI degrades the quality of the received signal and thus CCI impacts negatively on the system throughput. Considering again FIG. 1, it will be noted that there are crossover regions between adjacent sectors in FIG. 1. CCI in these locations will be high, and can be avoided by using different channels on overlapping sectors.
Another way to mitigate the CCI is to use antenna arrays at the BS, such antenna arrays being described for example in Chapter 3 of the publication “Smart Antennas, Adaptive Arrays, Algorithms, and Wireless Position Location”, edited by Dr T S Rappaport, IEEE, N.J. 1998, that chapter providing an introduction to smart antennas and spatial processing. An advanced (also referred to in the art as smart) antenna array consists of two or more closely spaced antennas, and in combination with a beamforming network, narrow beams with increased signal strength can be formed in the direction of the desired SS. Exploiting the spatial separation between users, the advanced antenna array can also reduce the interference to other SSs. The overall benefits of antenna arrays are increased range and improved signal strength (due to the antenna array gain), along with increased system capacity due to the efficient utilisation of spectral resources, i.e. reduced CCI.
One known type of smart antenna array is referred to as a fixed multi-beam antenna array system, where a finite number of fixed beams with predefined beam patterns and fixed pointing directions are employed. Another alternative type of smart antenna is the steered beam, or fully adaptive, antenna system. Unlike the fixed multi-beam systems, a steered beam system can radiate its energy in any direction, and in some cases can ensure little or no interference (nulling) in certain other directions. Typically, such steered beam systems are more complex to design than fixed multi-beam systems.
Considering the downlink communication from a BS (or more generically a point to multipoint device) to a particular SS (or more generically a telecommunications unit), the use of smart antenna arrays at the BS can enhance the instantaneous signal quality at the SS, but the random beam switching (and for steered beam systems, nulling) in the downlink communication will introduce random variations in the CCI, and consequently variations in the reported SINR that SSs will experience. There is an inherent delay between the time instance when a particular MCS is requested by an SS and the time instance when that SS is scheduled. If the CCI between the two instances varies significantly, then two possible outcomes can occur. Firstly, the SINR at the SS when scheduled could be higher, i.e. better, than anticipated. As a result, the requested MCS was too pessimistic. While error free transmission is more likely to occur, an alternative MCS with higher data rates could have been used instead. Alternatively, the SINR at the SS may be lower than anticipated. The requested MCS is in that case too optimistic, and the requested MCS may not be sufficiently robust enough to guarantee the desired QoS. This latter outcome is more dramatic, since many retransmissions will occur resulting in severe system throughput degradation.
Due to the complexity of design of steered beam systems, and the almost infinite variations in CCI levels that can result from the use of such steered beam systems, most systems employing smart antenna arrays for downlink communication have been based on fixed multi-beam systems. One such system is described in US 2005/0064872, which describes a technique for reducing shared downlink radio channel interference by transmitting to multiple mobiles using multiple antenna beams. In particular, a technique is described whereby abrupt time varying changes in the CCI are alleviated by scheduling to multiple mobile users using simultaneous multiple orthogonal beams per sector. The total base station power is split equally amongst the beams. Hence, if M simultaneous beams are used, then the received energy at the mobile will be reduced by M. Whilst this alleviates abrupt time varying changes in the CCI it gives rise to an elevated CCI level at all times, which impacts negatively on the system throughput.
An alternative approach also discussed in US 2005/0064872 is to retain the multiple orthogonal beams per sector, but to only use one beam at a time. To avoid rapid changes in the CCI, the beams are in such an embodiment switched at a very low rate, which gives time for the mobiles illuminated by the beam to report and experience approximately the same SINR when scheduled. When using such an approach, the above patent indicates that it is desirable to avoid synchronisation in the network in order to decrease the frequency of beam switching. A problem with the above-described approach is that for a few milliseconds after a beam switch, there are transient periods where the CCI will fluctuate rapidly. Another drawback with slow switched beam systems is that multi-user diversity is not fully exploited. Multi-user diversity is an inherent form of diversity present in all multi-user wireless communication systems. In the above-described system only parts of a sector are illuminated by a beam, and hence there may be users in other parts of the sector that experience constructive fading with signal quality that exceeds that of the signal quality of the scheduled user, but which cannot be scheduled in preference to that user due to not being illuminated by the beam in use at the time. Multi-user diversity seeks to exploit the time-varying nature of the quality of the channels by seeking to transmit data to users with the highest channel quality, thereby improving the overall system performance in terms of throughput. However, when using a slow switched beam system, it is not possible to make best use of the multi-user diversity.
Another problem with slow switched beam systems is the increase in latency resulting from the time taken to switch the beams, which can have an adverse effect on the QoS.
U.S. Pat. No. 6,438,389 describes a wireless communication system with adaptive beam selection. In particular, a wireless communication system is described having several antennas that are electronically controlled to form N distinct beams, and each one of the N beams is periodically measured for signal quality for each mobile subscriber. The two or more best beams are selected using switches controlled by a computer that stores and compares the signal quality measurements. The best beams are combined to produce a signal with improved quality.
US 2004/0235527 describes a system using multiple antenna beam base stations to provide re-use of communication channels. The system has base stations which utilise multiple beam antennas to provide a number of substantially non-overlapping antenna beams to provide directional wireless signal coverage in an area around an associated base station. Simultaneous use of channels within antenna beams of a single cell is facilitated through antenna beam isolation. A technique for reducing inter-cell interference is also discussed, where based on a number of modelling and/or empirical measurements, mutually exclusive antenna beam pairs between a “home” base station and the base stations surrounding the home base station are identified. Hence, for each beam of the home base station, beams in surrounding base stations that will experience interference if used at the same time are identified. The available resources, i.e. timeslots or frequencies, are apportioned amongst beam pairs according to their traffic needs, and a reference clock such as GPS is used to ensure synchronisation amongst the base stations of the network to ensure that beams that may interfere with each other are not used at the same time.
The above approach of producing a table of mutually exclusive beams based on modelling assumptions requires detailed two and possibly three dimensional path loss and shadow fading maps. Such maps can be obtained only through measurement reports from each and every subscriber station in the wireless network. In order to generate these maps the subscriber stations will also need to provide their co-ordinates. Managing and updating the maps is a cumbersome, memory intensive task.
Another disadvantage of the scheme proposed in US 2004/0235527 (and indeed of the earlier mentioned prior art techniques) is that the multiple antenna beam base stations described therein require calibrated arrays, which comprise of calibration units that estimate and correct any amplitude and phase distortions present in the transmit and/or receiver chain from the antenna elements down to baseband processing in the base station. Antenna architectures of this type are expensive to produce.
U.S. Pat. No. 6,804,521 describes a technique for reducing cross-beam interference in multi-beam wireless data transmission systems through temporal separation of the multiple beams. A section of a cell is geographically divided such that a number of beam patterns correspond to a number of user group areas. Downlink transmissions from a base station to a plurality of user terminals are then patterned such that transmissions to adjacent user group areas do not occur during the same time intervals. As shown for example in FIG. 8A of U.S. Pat. No. 6,804,521, sectors designated T1 transmit on their respective downlink paths during first time intervals, whilst sectors designated T2 transmit on their respective downlink paths during second time intervals, wherein the second time intervals do not overlap with the first time intervals. Whilst such an approach can reduce interference, CCI will still occur in sectors transmitting at the same time on the same channels. For example if the sector marked T2 in cell 810 that points north east uses the same channel as the sector marked T2 in cell 802 that points north east, then this latter sector will observe CCI resultant from the transmission from the aforementioned T2 sector in cell 810.
Accordingly, it would be desirable to provide a technique which enabled a further reduction in CCI within a wireless network.